In recent years, with a shrinking device for high integration of a semiconductor circuit, a demand for quality of silicon single crystal fabricated by a Czochralski method (which will be referred to as a CZ method hereinafter) that serves as a substrate of this circuit has been increased.
At the time of growing the silicon single crystal by the CZ method, oxygen of approximately 10 to 20 ppma (JEIDA: a conversion factor provided by Japan Electronic Industry Development Association is used) is usually dissolved into a melt from a quartz crucible and taken into the silicon single crystal at a silicon melt interface.
Then, in a process of cooling the silicon single crystal, the oxygen that has been taken into becomes a supersaturation state and agglomerates when a crystal temperature is reduced to 700° C. or less, and oxide precipitates (which will be also referred to as grown-in oxide precipitates) are formed. However, the size of the oxide precipitates is very small; therefore TZDB (Time Zero Dielectric Breakdown) characteristics as one of oxide dielectric breakdown voltage characteristics or device characteristics are not decreased on a shipping stage.
A defect caused due to single-crystal growth that decreases the oxide dielectric breakdown voltage characteristics or the device characteristics is a complex defect produced when a vacancy type point defect which is called a vacancy (which may be also referred to as Va hereinafter) and an interstitial silicon point defect which is called Interstitial-Si (which may be also referred to as I hereinafter) taken into the silicon single crystal from a silicon melt become supersaturated during cooling the crystal and they are agglomerated together with oxygen, and it has been revealed that it is a grown-in defect such as FPD, LSTD, COP, or OSF.
Before describing these defects, generally known matters about a factor that determines the concentration of each of Va and I that are taken into the silicon single crystal will be first described.
FIG. 5 is a view showing a defect region of a silicon single crystal when V/G, which is a ratio of a pulling rate V (mm/min) to an average value G (° C./mm) of a crystalline temperature gradient in a pulling shaft direction in a temperature range from a silicon melting point to 1300° C., is changed by varying V at the time of growing single crystal.
In general, a temperature distribution in single crystal is dependent on a CZ in-furnace structure (which will be referred to as a hot zone (HZ) hereinafter), and this distribution hardly varies even though a pulling rate is changed. Therefore, in the case of a CZ furnace having the same configuration, V/G is associated with a change in pulling rate alone. That is, the pulling rate V and V/G approximately has a relationship of direct proportion. Accordingly, an ordinate in FIG. 5 represents a pulling rate V.
In a region where the pulling rate V is relatively high, grown-in defects such as FPD, LSTD, or COP which can be considered as voids formed by agglomerating point defects called vacancies are densely present in the substantially entire region in a crystal radial direction, and the region where these defects are present is called a V-rich region.
When a growth rate is reduced, OSF rings produced in a crystal peripheral portion are contracted toward the inside of the crystal, and they are eventually annihilated. When the growth rate is further lowered, a neutral (which will be referred to as N hereinafter) region where deficiency or excess of vacancies or interstitial silicon hardly occurs appears. Although biased Va or I is present in this N region, it has saturated concentration or lower concentration, and hence the grown-in defect by agglomeration are not produced. This N region is divided into an Nv region where Va is dominant and an Ni region where I is dominant.
In the Nv region, it has been found out that, when a thermal oxidation treatment is effected, many oxide precipitates (bulk micro defects which will be referred to as BMDs hereinafter) are generated, and oxygen precipitation hardly occurs in the Ni region. Further, in a region where the growth rate is low, I becomes supersaturated, defects L/D (Large Dislocation: an abbreviation of an interstitial dislocation loop, e.g., LSEPD or LEPD) which can be considered as a dislocation loop generated as a result of assembling I are present at low density, and this region is called an I-Rich region.
Therefore, for example, when the single crystal pulled while controlling a growth rate so that the N region can be formed in the entire region in the radial direction from the center of the crystal is sliced and polished, a silicon substrate having the N region with greatly reduced defects can be obtained.
Moreover, when the above-described BMD is generated on the silicon substrate surface which is a device active region, device characteristics such as junction leakage are adversely affected but, on the other hand, if the BMD is present in a bulk other than the device active region, the BMD functions as a gettering site where a metal impurity mixed during the device process is captured, which is effective.
In recent years, as a method for forming BMD in a substrate having an Ni region where the BMD is hardly produced, a method for perform RTP (Rapid Thermal Process) (a rapid-heating and rapid-cooling heat treatment) has been suggested. This RTP treatment is a heat treatment method for rapidly increasing a temperature of a silicon substrate beyond a room temperature at a temperature increasing rate of, e.g., 50° C./second, heating and maintaining a temperature of approximately 1200° C. for approximately tens seconds, and rapidly effecting cooling.
A mechanism that the BMDs are formed when an oxide precipitate heat treatment is performed after the RTP treatment is described in Patent Literature 1 or Patent Literature 2 in detail. The BMD formation mechanism will now be briefly explained.
First, in the RTP treatment, Va are injected from a silicon substrate surface while a high temperature of 1200° C. is maintained in, e.g., an N2 atmosphere, and then redistribution and annihilation with I due to spread of Va occur during a reduction in temperature. As a result, a state that Va are ununiformly distributed in a bulk is realized. When the silicon substrate in such a state is subjected to a heat treatment at, e.g., 800° C., clustering of oxygen rapidly occurs in a region having high Va concentration, but clustering of oxygen does not occur in a region having low Va concentration. Then, when a heat treatment is performed at, e.g., 1000° C. for a given period of time, the clustered oxygen grows, and the BMDs are formed.
When the oxygen precipitation heat treatment is performed with respect to the silicon substrate after the RTP treatment in this manner, the BMDs having a distribution in a thickness direction of the silicon substrate are formed in accordance with a concentration profile of Va formed by the RTP treatment. Therefore, a desired Va concentration profile is formed on the silicon substrate by effecting the RTP treatment while controlling its conditions such as an atmosphere, a maximum temperature, a retention time, and others, and then the silicon substrate having a desired DZ width and a BMD profile in a thickness direction can be manufactured by performing the oxygen precipitation heat treatment with respect to the silicon substrate.
Further, Patent Literature 3 discloses that an oxide film is formed on a surface when the RTP treatment is effected in an oxygen gas atmosphere, I is injected from an oxide film interface, and hence BMD formation is suppressed. In this manner, the RTP treatment can both promote and suppress the BMD formation depending on conditions, e.g., an atmospheric gas, a maximum retention temperature, and others.
Such an RTP treatment is annealing in a very short time, outdiffusion of oxygen hardly occurs, and a reduction in oxygen concentration in a surface layer can be ignored.
Furthermore, it is known that, when an MOS transistor is fabricated in a device process and a reverse bias is applied to a gate electrode for its operation, a depletion layer spreads, but if BMDs are present in this depletion layer region, and presence of the BMDs in this depletion layer can cause junction leak. Therefore, grown-in defects as typified by COP, BMD, or grown-in oxide precipitates must not be present in a surface layer which is an operating region for many devices.
To annihilate defects concerning oxygen like COP, OSF nucleuses, or oxide precipitates, there is a method for lowering oxygen concentration to a solid solubility limit or less. For example, when a heat treatment is performed at 1100° C. or more and oxygen concentration in a surface layer is lowered to a solid solubility limit or less by utilizing outdiffusion of oxygen, the defects can be annihilated. However, since the oxygen concentration in the surface layer is considerably lowered due to outdiffusion of oxygen, there is a problem that mechanical strength of the surface layer is also lowered.
Additionally, to allow a semiconductor device to properly function, a minority carrier must have a sufficient lifetime. It is known that the lifetime of the minority carrier (which will be referred to as a lifetime hereinafter) is lowered by formation of a defect level due to metal impurities, oxygen precipitation, vacancies, and others. Therefore, to safely secure a function of a semiconductor device, a silicon substrate must be manufactured by a method that can realize a sufficient lifetime.
Therefore, in recent devices, a wafer with a sufficient lifetime in which grown-in defects or grown-in oxide precipitates concerning oxygen are not present in a device operating region and BMDs serving as gettering sites are sufficiently precipitated in a bulk outside the device operating region by a device heat treatment is effective.
Patent Literature 1 discloses a method for slicing out a silicon substrate having an entire plane of an N region from N-region single crystal in which agglomerates of Va or I are not present and performing an RTP treatment with respect to the silicon substrate. In case of this method, since grown-in defects are not present in Si which is a material, it can be considered that no problem occurs even though the RTP treatment is performed but, when TDDB (Time Dependent Dielectric Breakdown) characteristics as temporal breakdown voltage characteristics presenting long-term reliability of an oxide film are measured after preparing a silicon substrate having an entire plane of an N region and performing the RTP treatment, TZBD characteristics hardly lowered in an Nv region of the silicon substrate, but the TDDB characteristics may be lowered in some cases. Further, as disclosed in Patent Literature 4, since a region where the TDDB characteristics are lowered is an Nv region and also a region where defects detected by the RIB method (RIE defects) are present, development of a silicon substrate having no RIE defect present in a surface layer and a manufacturing method thereof is very important.
The RIE method will now be described.
As the RIE method, there has been known a method disclosed in Patent Literature 5 as a method for evaluating small crystal defects containing silicon oxide (which will be referred to as SiOx hereinafter) in a silicon substrate while providing a resolution in a depth direction. According to this method, highly selective anisotropic etching such as reactive ion etching (which will be referred to as RIE hereinafter) is performed with respect to a main surface of the substrate until a fixed thickness is provided, and a remaining etching residue is detected, thereby evaluating crystal defects. Since an etching rate differs depending on a crystal defect forming region containing SiOx and a non-forming region (the etching rate is low in case of the former one), when the etching is performed, a conical protrusion having a crystal defect containing SiOx at an apex remains on a main surface of the substrate. Since the crystal defect is emphasized in the form of a protrusion by the anisotropic etching, and hence a very small defect can be easily detected.
A method for evaluating a crystal defect disclosed in Patent Literature 5 will now be described.
An oxide precipitate is formed when oxygen dissolved in a silicon substrate in a supersaturated state is precipitated as SiOx by a heat treatment. Further, when anisotropic etching having a high selectivity ratio is performed with respect to BMDs included in the silicon substrate in a halogen-contained mixed gas (e.g., HBr/Cl2/He+O2) atmosphere with a commercial available RIE device, conical protrusions generated due to the BMDs are formed as etching residues (hillocks). Therefore, the crystal defects can be evaluated based on the hillocks. For example, counting the number of obtained hillocks enables obtaining density of the BMDs in the silicon substrate in the etched range.